MIRA can reduce expenses of running a node (full RPC, seed, witness) but at the cost of a much slower replay. On my witness test node, a replay usually takes 20-22 hours without MIRA but when I enabled MIRA I gave up after 3 days of replay still not completed. For a top witness, it is critical to be able to replay as fast as possible in order to get the node back on track on events such as a HardFork where a replay is required.

MIRA will help with reducing the costs of other nodes but for witnesses nodes it's not recommended for the reason above.